System and method for high speed satellite-based free-space laser communications using automatic gain control

ABSTRACT

A high speed satellite-based laser communications system and method for communications between a satellite-based transmitter system and a ground-based receiver over a free space optical link. The satellite-based transmitter system includes an encoder to encode data, a polarization modulator to linearly polarize the encoded data, one or at least two transmitters to transmit the laser beam, and a quarter-wave optical wave plate to circularly polarize the signal to be transmitted. The ground-based receiver includes an automatic gain control to apply AGC to the received data before the polarizations are reversed and the data is decoded. The system enables an increased data throughput and reduces or eliminates the effects of signal fading.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of U.S.Provisional Patent Application No. 62/342,454, filed on May 27, 2016,the entire contents of which are incorporated by reference as if fullyset forth herein.

FIELD

The present invention is generally related to an improved system andmethod for transmitting optical signals from a satellite to aground-based receiver. In particular, the present invention relates tohigh speed free-space laser communications between a satellite-basedtransmitter and a ground-based receiver that uses automatic gain controlto increase data throughput and reduce or eliminate the effects ofsignal fading.

BACKGROUND

Polarization shift keying (PolSK) modulation encodes data onto anoptical beam by altering the polarization state of the beam throughadjustment of optical phase between two polarization states. Thisencoding allows the transmission of data through a free-space optical(“FSO”) link using existing high-speed phase modulators. A free-spaceoptical link is a link that uses light propagating in free space, suchas air, outer space or a vacuum, to wirelessly transmit data fortelecommunications or computer networking. The optical transmission of asignal over a free space optical link presents challenges as the opticalsignal is subject to atmospheric turbulence, that causes the opticalsignal to fade.

A recent paper by Bai, et al. in Performance Analysis of PolarizationModulated Direct Detection Optical CDMA Systems over Turbulent FSO LinksModeled by the Gamma-Gamma Distribution, Photonics 2015, 2, 139-155,ISSN 2304-6732, describes optical transmissions between a ground-basedtransmitter and receiver over an FSO link when using polarization shiftkeying (PolSK) to modulate the optical signal along with optical codedivision multiple access (CDMA) encoding of the optical signal to betransmitted over the FSO link. The prior art system of Bai, el al.,shown in FIG. 1, illustrates both a transmitter 10 to process andtransmit data 11 as an optical signal over an FSO and a receiver 20 toreceive and process the data. Transmitter 10 illustrated in Bai, et al.includes a polarization controller 14 that linearly polarizes an inputlaser beam 18 at a 45° angle and feeds the polarized laser beam to amodulator 16. Modulator 16 includes a polarization beam splitter (PBS)16 a, an optical phase modulator 16 b for modulating CDMA-encoded dataonto the laser beam, and a polarization beam combiner 16 c that combinesthe phase modulated CDMA-encoded data and the unmodulated portion of thelaser beam for transmission as an optical signal over the FSO link 19.Transmitter 10 in Bai, et al. further includes a CDMA encoder 13 toencode both data 11 and a modified prime code (MPC) sequence for aparticular user which is then fed to modulator 16. Thus, the transmittedsignal is both CDMA-encoded and polarization modulated.

Receiver 20 in Bai, et al. reverses the polarization modulation and theCDMA encoding. Receiver therefore includes polarization controller 21 toreceive the optical signal, a second polarization beam splitter (PBS) 22splits the signal based on the polarization state of each portion of thesignal and reconstructs the data that was transmitted by applyingpolarizers 23, 24, optical correlators 25, 26, and photodetectors 27, 28to the split polarized signal, then adding and amplifying the signal atrespective adder 28 and amplifier 29, filtering the signal at low-passfilter (LPF) 30 and a decision processor 31 for determining signalreconstruction of the signal output from LPF 30.

The system described in Bai, et al. suffers from problems of signalfading over FSO links for high-speed laser communications from asatellite-based transmitter to a ground-based receiver where it isdesired to transmit signals at very high speeds such as at 10 Gbits/sec.A signal transmitted by a satellite-based laser communications systemmust necessarily pass over a long FSO link and therefore encounters agreat deal of fading. An improved solution is therefore needed forsatellite to ground laser communications to provide for faster, morereliable, and higher-quality laser communications that account forfading caused by atmospheric conditions.

Satellite-based laser communications systems generally require complexelectronics and electro-optical sub-systems to provide a robust systemthat can deal with atmospheric conditions. These electronics andelectro-optical sub-systems tend to undesirably occupy a significantamount of space, which, in turn, requires larger satellites. It istherefore desirable to develop electronics and electro-opticalsub-systems that are lighter, cheaper and more compact.

SUMMARY

A satellite-based laser communications system in accordance with thepresent invention is therefore provided with a receiver that includes anautomatic gain control to process the received polarization modulatedsignal before demodulating the received signal and thereby reduce oreliminate the effects of channel fading.

In accordance with an embodiment of the present invention, aground-based receiver for receiving a signal transmitted by asatellite-based transmitter subsystem of a satellite-based lasercommunications system for communication between a satellite and theground-based receiver using a laser beam over a free-space optical linkthat uses light propagating in free space for wireless datacommunications, includes (a) an optical automatic gain control circuitthat processes the received signal that was transmitted by thesatellite-based transmitter subsystem using the laser beam to accountfor signal fading and atmospheric conditions over the free-space opticallink. The optical automatic gain control circuit has a channel stateestimator that receives a fraction of the received signal, estimates astate of the communication channel parameters that may have degraded thereceived signal, and outputs a control signal comprising the estimatedcommunication channel parameters, and an optical amplifier to receivethe control signal that is output by the channel state estimator and toadjust and amplify the received signal based, at least in part, on thecontrol signal to output an automatic gain controlled signal that hastwo circularly polarized states.

The ground-based receiver in this embodiment of the present inventionfurther includes (b) a quarter-wave (λ/4) optical wave plate to convertthe automatic gain controlled signal from two circularly polarizedstates into an optical beam having two linear polarization states,including a first linear polarization state and a second linearpolarization state; (c) a polarizing beam splitter to split the opticalbeam into a first linearly polarized beam corresponding to the firstlinear polarization state and a second linearly polarized beamcorresponding to the second linear polarization state; (d) twodetectors, including a first detector to detect the first linearlypolarized beam and a second detector to detect the second linearlypolarized beam; (e) image processing circuitry or a computer-implementedimage processing module comprising an algorithm to generate a differencein the output of the two detectors to develop an output signal thatcomprises the signal as encoded at the satellite-based transmitter; (f)a decoder to decode the output signal to obtain the transmitted data;and (g) an output module to output the transmitted data, wherein theoptical automatic gain control circuit at least partially compensatesfor fading effects that occur during satellite transmissions to enableimprovement in data throughput. A ground-based receiver of the presentinvention enables the received signal to be transmitted by thesatellite-based transmitter subsystem to the ground-based receiver at adata rate at least as high as 10 Gbps.

In embodiments, the decoder at the ground-based receiver is configuredto perform error correction on the output signal when the receivedsignal was error correction encoded at the satellite-based transmittersubsystem. Also, in embodiments, the decoder at the ground-basedreceiver includes a deinterleaver to deinterleave the encoded outputsignal when the received signal was interleaved at the satellite-basedtransmitter subsystem. Further, in embodiments, the decoder at theground-based receiver includes a demultiplexer to obtain multiplechannels of data from the output signal when the multiple channels ofdata were multiplexed at the satellite-based transmitter subsystem.

In embodiments, the optical amplifier comprises one or more opticalfiber amplifiers. Further, in embodiments, the first detector isconfigured to detect a first area on an imaging sensor focal plane andthe second detector is configured to detect a second area on the imagingsensor focal plane. Also, in embodiments, the ground-based receiver maybe configured to be used in conjunction with an on-off keying signalingsystem or a differential phase shift keying (DISK) system.

In accordance with another embodiment of the present invention, asatellite-based laser communications system for communication between asatellite and a ground-based receiver comprises a satellite-basedtransmitter subsystem to transmit a signal to a ground-based receiverover a free-space optical link, wherein the free space optical link useslight propagating in free space, such as air, outer space or a vacuum,to wirelessly transmit data for telecommunications or computernetworking, and the ground-based receiver. The satellite-basedtransmitter subsystem includes (a) an input module for receiving data tobe transmitted to the ground-based receiver, (b) an encoder to encodethe data to be transmitted, (c) a processor configured to generate atransmission signal comprising the encoded data, (d) a laser lightsource to generate a linearly polarized laser beam, (e) at least onepolarization modulator that further encodes the encoded data in thetransmission signal onto the laser beam by polarization modulation ofthe laser beam through adjustment of an optical phase between two linearpolarization states using one or more high-speed phase modulators eachcomprising an electro-optical crystal aligned with its active axis at45° to the linearly polarized input beam, (f) at least one transmitterfor transmitting the polarization modulated laser beam, wherein, theamount of energy in each of the two linear polarization states isdependent on the applied voltage, and (g) a quarter-wave (λ/4) opticalwave plate to convert the two linear polarization states of thepolarization modulated laser beam into circularly polarized states inwhich to transmit the laser beam via the free-space optical link so thatthe ground-based receiver need not be aligned in rotation with respectto the transmitter.

In embodiments, the ground-based receiver of the satellite-based lasercommunications system includes (a) an optical automatic gain controlcircuit that processes the received signal to account for signal fadingand atmospheric conditions over the free-space optical link. The opticalautomatic gain control circuit has a channel state estimator thatreceives a fraction of the received signal and estimates a state ofcommunication channel parameters that may have degraded the receivedsignal, and outputs a control signal comprising estimated communicationchannel parameters to the optical amplifier, and has an opticalamplifier to receive the control signal that is output from the channelstate estimator and to adjust and amplify the received signal based, atleast in part, on the control signal to output an automatic gaincontrolled signal that has two circularly polarized states. Theground-based receiver further includes (b) a quarter-wave (λ/4) opticalwave plate to convert the automatic gain controlled signal from the twocircularly polarized states into an optical beam having two linearpolarization states, including a first linear polarization state and asecond linear polarization state, (c) a polarizing beam splitter tosplit the optical beam into a first linearly polarized beamcorresponding to the first linear polarization state and a secondlinearly polarized beam corresponding to the second linear polarizationstate, (d) two detectors, including a first detector to detect the firstlinearly polarized beam and a second detector to detect the secondlinearly polarized beam, (e) image processing circuitry or acomputer-implemented image processing module comprising an algorithm togenerate the difference in the output of the two detectors to develop anoutput signal that comprises the signal as encoded at thesatellite-based transmitter subsystem, (f) a decoder to decode theoutput signal to obtain the transmitted data; and (g) an output moduleto output the transmitted data. The optical automatic gain controlcircuit at least partially compensates for fading effects that occurduring satellite transmissions to enable improvement in data throughput.Thus, the signal is not overcome by interference like atmosphericscintillation when transmitted at high speeds from the satellite-basedtransmitter subsystem to the ground-based receiver.

In accordance with an alternative exemplary embodiment of the presentinvention, a satellite-based laser communications system forcommunication between a satellite and a ground-based receiver comprisesa satellite-based transmitter subsystem to transmit a signal to aground-based receiver over a free-space optical link, wherein the freespace optical link uses light propagating in free space, such as air,outer space, or a vacuum, to wirelessly transmit data fortelecommunications or computer networking, and the ground-basedreceiver. The satellite-based transmitter subsystem includes (a) aninput module for receiving data to be transmitted to the ground-basedreceiver, (b) an encoder to encode the data to be transmitted, (c) aprocessor configured to generate a transmission signal comprising theencoded data, (d) a laser light source to generate a linearly polarizedlaser beam, (e) a polarization modulator that further encodes theencoded data in the transmission signal onto the laser beam by alteringthe polarization state of the laser beam through adjustment of anoptical phase between two linear polarization states using one or morehigh-speed phase modulators each comprising an electro-optical crystalaligned with its active axis at 45° to the linearly polarized inputbeam; wherein the amount of energy in each of the two linearpolarization states is dependent on the applied voltage, (f) at leasttwo transmitters, wherein each of the two transmitters transmits aportion of the polarization modulated laser beam that corresponds to arespective one of the two linear polarization states, and (g) aquarter-wave (λ/4) optical wave plate to convert the two linearpolarization states into circularly polarized states in which totransmit the polarization modulated laser beam via the free-spaceoptical link so that the ground-based receiver need not be aligned inrotation with respect to the transmitter.

In accordance with this alternative exemplary embodiment, theground-based receiver comprises (h) an optical automatic gain controlcircuit that processes the received signal to account for signal fadingand atmospheric conditions over the free-space optical link, the opticalautomatic gain control circuit comprising a channel state estimator thatreceives a fraction of the received signal and estimates a state ofcommunication channel parameters that may have degraded the receivedsignal, and outputs a control signal comprising estimated communicationchannel parameters to the optical amplifier, and an optical amplifier toreceive the control signal that is output from the channel stateestimator and to adjust and amplify the received signal based, at leastin part, on the control signal to output an automatic gain controlledsignal that has two circularly polarized states. The ground-basedreceiver further comprises (i) a quarter-wave (λ/4) optical wave plateto convert the automatic gain controlled signal with the circularlypolarized states transmitted with the received laser beam back into anoptical beam having two linear polarization states, (j) a polarizingbeam splitter to split the optical beam into two beams, one for each ofthe two linear polarization states, (k) two detectors, one for each ofthe two beams, to capture the linear polarization state for the beam tobe detected, (l) circuitry or an image processing module comprising analgorithm to generate the difference in the output of the two detectorsor focal plane areas to develop an output signal that comprises thesignal as encoded at the satellite-based transmitter subsystem, (m) adecoder to decode the output signal to obtain the transmitted data, and(n) an output module to output the transmitted data.

In the alternative embodiment of the satellite-based lasercommunications system, the satellite-based transmitter subsystem maycomprise at least two transmitters that use a time division diversityscheme to transmit the laser beam to the ground-based receiver toaccount for possible different arrival times at the ground-basedreceiver for different channels.

In embodiments of the satellite-based laser communications system, thepolarization modulation boosts signal strength and is not overcome byinterference like atmospheric scintillation and the automatic gaincontrol increases data throughput and eliminates the effects of fadingthat occur during satellite transmissions.

In embodiments, the encoder may be a code division multiple access(CDMA) encoder that is configured to (1) encode a synchronizationchannel using asynchronous CDMA encoding with pseudo-random modulation,and (2) separately encode the data to be transmitted as data symbolsusing CDMA encoding with modified Walsh matrix modulation to distributethe encoded data symbols across multiple channels so as to maximize adata transfer rate; wherein, the modified Walsh matrix modulation valuesare derived from the pseudo-random modulation vector and Walsh vectorwithin each data symbol by successively projecting each Walsh vector outof the sub-space spanned by the pseudo-random modulation vector and anyprevious modified Walsh vectors, and then normalizing. Further, inembodiments, the decoder at the ground-based receiver comprises a CDMAdecoder to decode the received multiple channels of the CDMA-encodedsignal using the synchronization signal to obtain the transmitted data.Use of CDMA encoding may further increase data throughput and reducefading effects on the CDMA-encoded signal that results from thesatellite transmissions over the free space optical link.

In embodiments, the satellite-based transmitter subsystem includes anerror correction encoder to encode the CDMA-encoded data with errorcorrection codes, and the ground-based receiver includes an errorcorrection decoder to decode the error correction coding for theCDMA-encoded data. Also, in embodiments, the satellite-based transmittersubsystem further includes an interleaver to interleave the CDMA-encodeddata, and the ground-based receiver further includes a deinterleaver torestore the CDMA-encoded data.

A relative intensity between the two linear polarization states may beused by the encoder to encode the data to be transmitted into an analogor digital format. Moreover, in embodiments, a ratio of energy betweenthe two linear polarization states is not affected by atmosphericscintillation, and can thus be used to send more than one bit of digitalinformation, or analog information, independent of atmosphericscintillation or link transmission properties.

In embodiments, the optical amplifier of the ground-based receiver maycomprise one or more optical fiber amplifiers. Also, in embodiments, afirst detector of the two detectors is configured to detect a first areaon an imaging sensor focal plane and a second detector of the twodetectors is configured to detect a second area on the imaging sensorfocal plane.

Moreover, in embodiments, the ground-based receiver may also be used inconjunction with OOK signaling systems (on-off keying or OOK) or inconjunction with Differential phase shift keying (DPSK) systems.

The present invention further includes a method of performing automaticgain control on a polarization modulated signal at the input to aground-based receiver in accordance with any of the embodiments of thepresent invention.

In accordance with embodiments of the present invention, a method ofprocessing a signal received at a ground-based receiver from asatellite-based transmitter subsystem of a satellite-based lasercommunications system, includes (a) receiving, by the ground-basedreceiver, the signal that has been transmitted using a laser beam over afree-space optical link using light propagating in free space forwireless data communications, wherein the transmitted signal waspolarization modulated onto the laser beam by altering the polarizationstate of the laser beam through adjustment of an optical phase betweentwo linear polarization states, including a first linear polarizationstate and a second linear polarization state, using one or morehigh-speed phase modulators each comprising an electro-optical crystalaligned with its active axis at 45° to the linearly polarized inputbeam, and wherein the two linear polarization states of the polarizationmodulated laser beam were converted into two circularly polarized statesfor transmission using a quarter-wave (λ/4) optical wave plate. Themethod further includes: (b) performing, by an optical automatic gaincontrol circuit, automatic gain control on the received signal at aninput to the ground-based receiver to account for signal fading andatmospheric conditions over the free-space optical link, the performanceof optical automatic gain control comprising (1) estimating, using achannel state estimator, a state of communication channel parametersthat may have degraded the received signal and outputting a controlsignal comprising estimated communication channel parameters to anoptical amplifier; and (2) adjusting and amplifying, using the opticalamplifier, the received signal based, at least in part, on the controlsignal output to output an automatic gain controlled signal that has thetwo circularly polarized states. The method further includes (c)converting the automatic gain controlled signal from the two circularlypolarized states into an optical beam having two linear polarizationstates using a quarter-wave (λ/4) optical wave plate, (d) splitting,with a polarizing beam splitter, the optical beam into a first linearlypolarized beam corresponding to the first linear polarization state anda second linearly polarized beam corresponding to the second linearpolarization state, (e) detecting the first linearly polarized beamusing a first detector and detecting the second linearly polarized beamusing a second detector, (f) generating, using image processingcircuitry or a computer-implemented image processing module, adifference in the output of the two detectors to develop an outputsignal that comprises the signal as encoded at the satellite-basedtransmitter, (g) decoding, using a decoder, the output signal to obtainthe transmitted data, and (h) outputting the decoded data. The method atleast partially compensates for fading effects that occur duringsatellite transmissions to enable improvement in data throughput.

In embodiments, the detection of the first linear polarization state andthe second polarization state of the two optical beams comprisesdetecting a first area on an imaging sensor focal plane using the firstdetector and detecting a second area on the imaging sensor focal planeusing the second detector.

In embodiments, the method further comprises performing error correctionon the output signal where the transmitted signal was error correctionencoded. Also, in embodiments, the method comprises performingdeinterleaving on the output signal when the transmitted signal wasinterleaved.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described withreferences to the accompanying figures, wherein:

FIG. 1 is a diagram illustrating a prior art free-space lasercommunications system for transmission of an optical signal that hasbeen polarization modulated and CDMA-encoded over an FSO link;

FIG. 2 is a diagram illustrating a high-speed free-space lasercommunications system for satellite-to-ground communications inaccordance with an embodiment of the present invention;

FIG. 3 is a diagram illustrating a high-speed free-space lasercommunications system for satellite-to-ground communications inaccordance with a second embodiment of the present invention;

FIG. 4 is a block diagram of an automatic gain control circuit inaccordance with an embodiment of the present invention for use in ahigh-speed free-space laser communications;

FIG. 5 is a diagram illustrating a variation of the first embodiment ofthe high-speed free-space laser communication system shown in FIG. 2;

FIG. 6 is a diagram illustrating a variation of the second embodiment ofthe high-speed free-space laser communication system shown in FIG. 3;and

FIG. 7 is a flow chart illustrating a method of processing a signalreceived at a ground-based receiver from a satellite-based transmittersubsystem of a satellite-based laser communications system in accordancewith an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

The present invention is generally related to an improved system andmethod for transmitting optical signals from a satellite to aground-based receiver. In particular, the present invention relates tohigh speed free-space laser communications between a satellite-basedtransmitter and a ground-based receiver that uses automatic gain controlto increase data throughput and reduce or eliminate the effects ofsignal fading.

A system and method in accordance with the present invention provides ahigh-speed free-space laser communications that at the transmitterpolarization modulates an optical signal to be transmitted over an FSOlink into two orthogonal quarter-wave circularly polarized states and atthe receiver applies an automatic gain control to the optical signalthat has been received at the receiver while the received optical signalis circularly polarized and before the signal is polarizationdemodulated.

FIG. 2 is a diagram of the satellite-based laser communications system 1of the present invention that comprises a satellite-based PolSKtransmitter 40 in a transmitter subsystem (input device 42 to wave plate54) to transmit data over an FSO link and a ground-based receiver 41that is configured to receive and process the transmitted data. Incommunications system 1, data is input via an input device/input module42 at transmitter 40. The input device/input module 42 collects inputdata from data sources and formats the collected data, as necessary, forinput to the transmitter subsystem. The input data is then encoded atencoder 44 by an encoding scheme, such as, for example, an optical CDMAencoding scheme. Encoder 44 may have optional circuitry to perform oneor more operations on the data to format the data and improve itsrobustness. For example, encoder 44 may include an optional ECC encoderto add error correction codes to the encoded data to allow for errorcorrection to be performed at receiver 41, and may be optionallyinterleaved at an interleaver to enhance the ability to perform errorcorrection, especially when there are burst errors. Encoder 44 may alsoreformat the data, multiplex multiple channels of data, perform timedivision diversity, and allow for multiple accesses. A processor 46coupled to encoder 44 is configured to generate a transmission signalcomprising the encoded data.

The encoded data output from encoder 44 is then modulated onto alinearly polarized laser beam that is generated by a laser light source50. A polarization modulator 52, which may be included in transmitter40, alters the polarization state of a portion of the laser beam throughadjustment of an optical phase between two orthogonal linearpolarization states, where the amount of energy in each of the twolinear polarization states is dependent on the applied voltage. Thevoltage that is applied for each of the two linear polarization statesmay be varied such that the relative intensity between the two linearpolarization states can also be used to encode additional informationinto the signal using another analog or digital format. The polarizationof the laser beam into the two linear polarization states is performedby an electro-optical crystal that is aligned with its active axis at45° to output the linearly polarized input beam.

After the laser beam is linearly polarized into two states, the laserbeam is transmitted by transmitter 40 through a quarter-wave (λ/4)optical wave plate 54 to convert the two linear polarization states intocircularly polarized states and the laser beam is then transmitted viathe free-space optical link 19 to the ground-based receiver 41. Bytransforming the linearly polarized states into circularly polarizedstates, the receiver 41 need not be aligned in rotation with respect tothe transmitter 40 to receive the transmitted optical signal.

The polarization modulation receiver can be used in conjunction with OOK(on-off keying) signaling systems to convey additional information or inconjunction with DPSK (differential phase shift keying) systems tomodulate additional information onto the transmitted optical signal.

Receiver 41 receives the optical signal in the form of a laser beamtransmitted over FSO link 56 and subjects the signal to an opticalautomatic gain control (AGC) circuit 58. FIG. 4 shows an implementationof the AGC circuit 58. In FIG. 4, the optical automatic gain controlcircuit 58 processes the received signal to account for signal fadingand atmospheric conditions over the free-space optical link 56. Theoptical automatic gain control circuit 58 includes an optical amplifier80, such as one or more optical fiber amplifiers, to adjust and amplifythe received signal in the laser beam to output an automatic gaincontrolled signal, and a channel state estimator 81. Channel stateestimator 81 captures a fraction of the incoming channel and uses it toestimate the communication channel parameters, particularly the fadestate of the channel, that degraded the received signal as it passedthrough the free-space optical link 56. Control signals 82, comprisingthe estimated communication channel parameters, are sent to the opticalamplifier 80 where they are used to adjust and amplify the signal outputfrom automatic gain control 58.

Significantly, the optical automatic gain control circuit 58 limits thebandwidth range of the received laser beam signal to be closer to thebandwidth of the signal that was actually transmitted so that detectors64, 66 at receiver 41 can process a signal having a more limitedbandwidth. For the AGC circuit 58 to work effectively, the fade ratemust be slower than the data rate, as is the case in high-speedsatellite communications where fast data rates, such as data rates atleast as high as 10 Gb/sec (Gbps), are desired.

After performing automatic gain control, the signal is transmittedthrough a quarter-wave (λ/4) optical wave plate 60 to transform thesignal from a circularly polarized state back to a linearly polarizedsignal. Next, a polarizing beam splitter 62 splits the beam into twobeams, one for each of the two linear polarization states. Each of thebeams is input to a respective detector 64, 66, to recapture the entiresignal and the outputs are fed to an image processing circuit thatincludes circuitry, such as comparator 68, or a computer-implementedimage processing module (not shown) using an image processing algorithm,to generate the difference in the output of the two detectors 64, 66,that detect separate areas on an imaging focal plane, to develop anoutput signal that comprises the encoded signal as encoded at thesatellite-based transmitter subsystem.

The output of the comparator 68 may then be optically amplified, such asoptical fiber amplifier 70, and may then be decoded at decoder 76.Decoder 76 performs operations that are the inverse to the operationsthat were performed at encoder 44. For example, if CDMA encoding wasused at the encoder 44, then signal must be CDMA decoded at decoder 76.If interleaving was performed at encoder 44, decoder 76 must performdeinterleaving and an ECC encoded signal must be ECC decoded. Thetransmitted data is output via an output module 78 where the data may bereformatted as necessary for transmission to users via a network, suchas a telecommunications network or the Internet.

As noted above, in embodiments, encoder 44 may perform optical CDMAencoding. The optical CDMA coding may be performed for example, byencoding a synchronization channel using asynchronous CDMA encoding withpseudo-random modulation, and separately encoding the data to betransmitted as data symbols using CDMA encoding with modified Walshmatrix modulation to distribute the encoded data symbols across multiplechannels so as to maximize a data transfer rate. The modified Walshmatrix modulation values are derived from the pseudo-random modulationvector and Walsh vector within each data symbol by successivelyprojecting each Walsh vector out of the sub-space spanned by thepseudo-random modulation vector and any previous modified Walsh vectors,and then normalizing the result. The multiple channels and the separatesynchronization channel may be multiplexed into the transmission signalby direct addition of each channel's signal value, and the transmissionsignal may be normalized by scaling the multiplexed signal values to thefull modulation range within each pseudo-random period.

For CDMA decoding, the received multiple channels of the CDMA-encodedsignal uses the synchronization signal to obtain the transmitted data.Using CDMA encoding and decoding as well as the error correctionprocessing and interleaving, further reduces interference and fading inthe satellite transmission and increases data throughput. However, theCDMA transmitted signal nevertheless may still be subject to fadingduring transmission that is not compensated for by the CDMA encoding orother encoding steps. Thus, the automatic gain control circuit 58 at theinput to the receiver 41 beneficially compensates for the fading effectsof satellite transmissions, increases the data throughput via the FSOlink 56 and boosts throughput. As the ratio of energy between the twopolarization states in a satellite-based laser communications system ofthe present invention is not affected by atmospheric scintillation, theratio of energy can also be used to send more than one bit of digitalinformation, or analog information, independent of atmosphericscintillation or link transmission properties.

FIG. 3 shows an alternative embodiment of a satellite-based lasercommunications system 100 that includes two transmitters 40′ and 40″,rather than just one transmitter 40, with polarization modulator 52preceding transmitters 40′ and 40″ and a multiplexer 53 between laser 50and optical wave plate 54. The other elements of system 100 are similarto the elements of system 1 shown in FIG. 2 and are identified bysimilar reference numerals. In the embodiment of FIG. 3, polarizationmodulator 52 may have two phase modulators, which polarizes the laserbeam into one of the two orthogonal linear polarization states using anelectro-optical crystal that is aligned with its active axis at 45° tothe input linearly polarized input beam. The output of each phasemodulator is input to one of two transmitters 40′ and 40″. The signalsfrom the two phase modulators are then multiplexed at multiplexer 52 andinput to quarter-wave (λ/4) optical wave plate 54 to convert the twolinear polarization states into circularly polarized states. Thecircularly polarized laser beam is then transmitted via the free-spaceoptical link 19 from transmitters 40′ and 40″ to the ground-basedreceiver 41. The use of two transmitters increases the transmissionspeed and throughput of the optical signal through satellite-based lasercommunications system 100 to ground-based receiver 41.

FIG. 5 is a diagram illustrating a variation of the first embodiment ofthe high-speed free-space laser communication shown in FIG. 2. In FIG.5, polarization modulator 52 precedes transmitter 40, but the elementsshown in FIG. 5 are otherwise identical to FIG. 2.

FIG. 6 is a diagram illustrating a variation of the first embodiment ofthe high-speed free-space laser communication shown in FIG. 3. In FIG.6, separate polarization modulators 52′, 52″ are used to feed respectivetransmitters 40′, 40″, but the figure is otherwise identical to FIG. 3.

The use of an AGC circuit at the input to a receiver to address thefading of signals transmitted by satellite obviates the need foralternative circuitry/processing at the satellite-based transmitter formore intensive encoding and error correction. As a result, thetransmitter design may be simplified and made more compact, allowing thesize of the satellite can be reduced so that it is lighter, cheaper andmore compact. Moreover, space may be freed up in the satellite for otherequipment that provides additional functionality.

FIG. 7 illustrates a method of processing a signal received at aground-based receiver from a satellite-based transmitter subsystem of asatellite-based laser communications system in accordance with anexemplary embodiment of the present invention, such as the embodimentsof the ground-based receiver and the satellite-based lasercommunications systems described above. The ground-based receiverreceives, at step 90, the signal that has been transmitted using a laserbeam over a free-space optical link using light propagating in freespace for wireless data communications, wherein the transmitted signalwas polarization modulated onto the laser beam by altering thepolarization state of the laser beam through adjustment of an opticalphase between two linear polarization states, including a first linearpolarization state and a second linear polarization state, using one ormore high-speed phase modulators each including an electro-opticalcrystal aligned with its active axis at 45° to the linearly polarizedinput beam, and wherein the two linear polarization states of thepolarization modulated laser beam were converted into two circularlypolarized states for transmission using a quarter-wave (λ/4) opticalwave plate. At step 91, an optical automatic gain control circuitperforms automatic gain control on the received signal at an input tothe ground-based receiver to account for signal fading and atmosphericconditions over the free-space optical link, the performance of opticalautomatic gain control including (1) estimating, using a channel stateestimator, a state of communication channel parameters that may havedegraded the received signal and outputting a control signal includingestimated communication channel parameters to an optical amplifier; and(2) adjusting and amplifying, using the optical amplifier, the receivedsignal based, at least in part, on the control signal output to outputan automatic gain controlled signal that has the two circularlypolarized states. At step 92, the automatic gain controlled signal isconverted from the two circularly polarized states into an optical beamhaving two linear polarization states using a quarter-wave (λ/4) opticalwave plate. At step 93, a polarizing beam splitter splits the opticalbeam into a first linearly polarized beam corresponding to the firstlinear polarization state and a second linearly polarized beamcorresponding to the second linear polarization state. At step 94, afirst detector detects the first linearly polarized beam using a firstdetector and a second detector detects the second linearly polarizedbeam using a second detector. In embodiments, the detection of the firstlinear polarization state and the second polarization state of the twooptical beams includes detecting a first area on an imaging sensor focalplane using the first detector and detecting a second area on theimaging sensor focal plane using the second detector.

At step 95, image processing circuitry or a computer-implemented imageprocessing module is used to generate a difference in the output of thetwo detectors to develop an output signal that includes the signal asencoded at the satellite-based transmitter. At step 96, a decoderdecodes the output signal to obtain the transmitted data. At step 97,the decoded data is output. The method at least partially compensatesfor fading effects that occur during satellite transmissions to enableimprovement in data throughput.

In embodiments, the method further includes performing error correctionon the output signal where the transmitted signal was error correctionencoded. Also, in embodiments, the method includes performingdeinterleaving on the output signal when the transmitted signal wasinterleaved.

While particular embodiments of the present invention have been shownand described in detail, it would be obvious to those skilled in the artthat various modifications and improvements thereon may be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such modifications andimprovements that are within the scope of this invention.

1.-20. (canceled)
 21. A ground-based receiver for receiving a signaltransmitted by a satellite-based transmitter subsystem of asatellite-based laser communications system for communication between asatellite and the ground-based receiver using a laser beam over afree-space optical link that uses light propagating in free space forwireless data communications, wherein the received signal has beentransmitted as a circularly polarized signal, wherein the ground-basedreceiver comprises: (a) an optical automatic gain control circuit thatprocesses the received signal that was transmitted by thesatellite-based transmitter subsystem using the laser beam to accountfor signal fading and atmospheric conditions over the free-space opticallink, wherein the optical automatic gain control circuit comprises: (1)an optical amplifier to amplify the received signal to output anautomatic gain controlled signal that has two circularly polarizedstates; (b) a quarter-wave (λ/4) optical wave plate to convert theautomatic gain controlled signal from two circularly polarized statesinto an optical beam having two linear polarization states, including afirst linear polarization state and a second linear polarization state;(c) a polarizing beam splitter to split the optical beam into a firstlinearly polarized beam corresponding to the first linear polarizationstate and a second linearly polarized beam corresponding to the secondlinear polarization state; (d) image processing circuitry or acomputer-implemented image processing module comprising an algorithm togenerate a difference between the first linearly polarized beam and thesecond linearly polarized beam to develop an output signal thatcomprises the signal as encoded at the satellite-based transmittersubsystem; (e) a decoder to decode the output signal to obtain thetransmitted data; and (f) an output module to output the transmitteddata.
 22. The ground-based receiver of claim 21, wherein the decoder atthe ground-based receiver is configured to perform error correction onthe output signal when the received signal was error correction encodedat the satellite-based transmitter subsystem.
 23. The ground-basedreceiver of claim 21, wherein the decoder at the ground-based receivercomprises a deinterleaver to deinterleave the encoded output signal whenthe received signal was interleaved at the satellite-based transmittersubsystem.
 24. The ground-based receiver of claim 21, wherein thedecoder at the ground-based receiver comprises a demultiplexer to obtainthe multiple channels of data from the output signal when the multiplechannels of data were multiplexed at the satellite-based transmittersubsystem.
 25. The ground-based receiver of claim 21, wherein theoptical amplifier comprises one or more optical fiber amplifiers. 26.The ground-based receiver of claim 21, wherein the ground-based receiveris configured to be used in conjunction with an on-off keying signalingsystem.
 27. The ground-based receiver of claim 21, wherein theground-based receiver is configured to be used in conjunction with adifferential phase shift keying (DPSK) system.
 28. The ground-basedreceiver of claim 21, wherein the received signal has been transmittedby the satellite-based transmitter subsystem to the ground-basedreceiver at a data rate at least as high as 10 Gbps.
 29. A method ofprocessing a signal received at a ground-based receiver from asatellite-based transmitter subsystem of a satellite-based lasercommunications system wherein the received signal has been transmittedas a circularly polarized signal, the method comprising: (a) receiving,by the ground-based receiver, the received signal that has beentransmitted using a laser beam over a free-space optical link usinglight propagating in free space for wireless data communications,wherein the signal, as transmitted, was polarization modulated onto thelaser beam by altering the polarization state of the laser beam throughadjustment of an optical phase between two linear polarization states,including a first linear polarization state and a second linearpolarization state, using one or more high-speed phase modulators eachcomprising an electro-optical crystal aligned with its active axis at45° to the linearly polarized input beam, and wherein the two linearpolarization states of the polarization modulated laser beam wereconverted into two circularly polarized states for transmission using aquarter-wave (λ/4) optical wave plate; (b) performing, by an opticalautomatic gain control circuit, automatic gain control on the receivedsignal at an input to the ground-based receiver to account for signalfading and atmospheric conditions over the free-space optical link, theperformance of optical automatic gain control comprising: (1)amplifying, using the optical amplifier, the received signal to outputan automatic gain controlled signal that has the two circularlypolarized states; (c) converting the automatic gain controlled signalfrom the two circularly polarized states into an optical beam having thetwo linear polarization states using a quarter-wave (λ/4) optical waveplate; (d) splitting, with a polarizing beam splitter, the optical beaminto a first linearly polarized beam corresponding to the first linearpolarization state and a second linearly polarized beam corresponding tothe second linear polarization state; (e) detecting the first linearlypolarized beam and detecting the second linearly polarized beam; (f)generating, using image processing circuitry or a computer-implementedimage processing module, a difference between the first linearlypolarized beam and the second linearly polarized beam that have beendetected to develop an output signal that comprises the signal asencoded at the satellite-based transmitter subsystem; (g) decoding,using a decoder, the output signal to obtain the transmitted data; and(h) outputting the decoded data; wherein the method at least partiallycompensates for fading effects that occur during satellite transmissionsto enable improvement in data throughput.
 30. The method of claim 29,wherein the signal, as transmitted, was error correction encoded, andwherein the method further comprises performing error correction on theoutput signal.
 31. The method of claim 29, wherein the signal, astransmitted, was interleaved, and wherein the method further comprisesperforming deinterleaving on the output signal.